The gram-positive organism Staphylococcus aureus colonizes the nasal passages in 20-50 percent of people, and community-acquired pneumonia caused by methicillin-resistant S. aureus (MRSA) is increasing. This suggests a need to develop vaccination and/or treatment strategies. Toll-like receptor 2 (TLR2) recognizes peptidoglycan and initiates natural immunity to S. aureus. Dendritic cells respond to TLR ligands by producing many factors including IL-1b and IL-23. These cytokines condition T cells to produce IL-17 and IL-22 during a process called Th17 differentiation. Interleukin-17 and IL- 22 were recently shown to control lung S. aureus infection (Kudva, Scheller, et al; J Immunol, in press). It is not known which cell type(s) must express TLR2 for efficient production of IL-1b and IL-23 in vivo, or for clearance of MRSA. Since airway epithelial cells (AECs) express TLRs and are the first point of contact with lung pathogens, they could play a role in initiating inflammation. Interleukin-22 has multiple effects on epithelial cells including proliferation, survival, and induction of antimicrobial proteins. Intestinal epithelial cells respond to IL-22 by producing regenerating islet-derived 3 (Reg3) family genes, which can directly inhibit the growth of MRSA (preliminary data). In the lung, the IL-22 receptor (IL-22R1) is restricted to AECs, and exposure to MRSA increased Reg3 expression (preliminary data). These data led to the hypothesis that TLR2 signaling in AECs initiates Th17 cell recruitment to the lung during MRSA infection, and also synergizes with IL-22R to mediate host defense through the induction of Reg3 antimicrobial lectins. This hypothesis will be tested with the following aims: Specific Aim 1: Determine the roles of TLR2 and IL-22R1 expression in AECs during lung infection with S. aureus USA300. TLR2 expression in radio-resistant cells such as AECs will be studied using bone marrow chimeras between C57BL/6 and TLR2-/- mice. The role of IL-22R1 in AECs will be determined by crossing epithelial-specific-Cre mice with IL22R1-floxed mice. These experiments will establish the complementary roles of TLR2 and IL-22R1 signaling in AECs during MRSA infection. Specific Aim 2: Determine the role of Reg3 family lectins as possible effectors downstream of IL-22R. In vitro experiments will determine if TLR ligands and IL-22 synergize to induce Reg3 in tracheal epithelial cells. In vivo experiments will examine the role of Th17-derived cytokines in Reg3 expression. Reg3g-/- mice will be used to clarify its role in controlling MRSA infection.